Turbo, Siegbahn, Holweck and Gaede are known vacuum pumping mechanisms. Such mechanisms are driven by a motor which causes rotation about an axis. It is often desirable to provide a vacuum pump which is compact in size and in order to achieve a compact design, the motor is positioned differently relative to the various different types of pumping mechanisms in order to increase the efficient use of space inside the pump. For instance, as shown schematically in FIG. 8, a Holweck pumping mechanism comprises at least one cylinder 80 rotatable about an axis 82. A space is provided radially inward of the cylinder in which a motor 84 may be positioned provided that the outer diameter of the motor is constrained so that it fits within the space. As the Holweck cylinder is relatively long, an axial length of the motor is relatively less constrained. As shown schematically in FIG. 9, a Gaede or Siegbahn pumping mechanism comprises at least one disk 86 rotatable about the axis 82. Typically, a motor 88 for such mechanisms is positioned on one axial side of the mechanism and therefore, it is desirable to limit the axial length of the motor in order to conserve space in the pump. Conversely, a radial dimension of a motor for a Gaede or Siegbahn mechanism is relatively less constrained.
Motors for vacuum pumps are either constrained in axial or radial dimension according to the type of pumping mechanism with which they are used, and therefore it is necessary to provide two different types of motor. A motor of a compact size is desirable for use with both Holweck type and Siegbahn and Gaede types of vacuum pumping mechanisms.
Vacuum pumps may be used with scientific or manufacturing equipment which is sensitive to magnetic interference. Such equipment includes electron microscopes, focused ion beam instruments and lithography equipment. Turbomolecular pumps of the types described above are often used to achieve the high vacuum typically required in such equipment. A Turbomolecular pump 90 is shown in FIG. 10. Such pumps are designed for high rotational speeds in the region of 36,000 to 90,000 rpm and may be driven by a permanent magnet brushless DC motor 92. Such motors have a two-pole configuration in order to minimise the commutation frequency and simplify the design of the drive electronics. Brushless DC motors generally comprise one or more hall effect sensors 94 for sensing rotation of a permanent magnet rotor 96. The rotor 96 extends axially beyond an end of the stator 98 so that the sensors can measure the rotation of the rotor. The extension of the rotor causes increased stray magnetic fields to leak out of the pump and interfere with scientific or manufacturing equipment, even though typically the extension may be only around 5 mm. It is desirable to reduce magnetic interference from the motor.